Hydraulic systems are utilized in many forms of construction equipment such as hydraulic excavators, backhoe loaders, and end loaders. The equipment is usually mobile having either wheels or track and includes a number of hydraulically actuated devices such as hydraulic cylinders and motors. In most cases the hydraulic circuits are controlled by a parallel valve arrangement in which a hydraulic pump provides pressurized fluid to a plurality of hydraulic valves each associated with a hydraulic cylinder or motor. As an operator manipulates control levers located in the operator's compartment, hydraulic valves are controllably opened and closed such that pressurized fluid is controllably directed to the desired cylinder or motor.
If two such hydraulic valves connected in a parallel arrangement are opened simultaneously, the amount of fluid flowing through each of the valves is dependent upon the relative pressures in each fluid circuit and the relative size of the openings of each valve. In many situations, however, it is desirable to give priority to one particular cylinder or motor that would ordinarily not receive a high flow rate when operated simultaneously with low pressure circuits.
For example, if the control valve for the swing motor on an excavator is being operated at the same time that the stick cylinder is operated, it is advantageous to give priority to the swing motor. This is because the operator is most likely working on the sidewall of a trench and therefore requires a high force to be applied to the sidewall. To achieve the desired effect, the hydraulic system should automatically give hydraulic flow priority to the swing motor by decreasing the flow directed to the stick cylinder. Similarly, if both the travel motor and the boom are being operated, it is advantageous to give priority to the travel motor.
The present invention is directed at overcoming one or more of the problems as set forth above.